Visual illusions in computed tomography: phenomena related to Mach effect.

Mach bands, a visual phenomenon resulting from lateral inhibitory impulses in the retina of the eye, are recognized under a variety of circumstances as lucent or dense lines due to overlap of radiographic shadows. A similar phenomenon, the background contrast effect, commonly occurs in computed tomography because of the effect of background density on a particular structure. This may be a source of diagnostic error. To avoid this problem, measurement of the densities of pertinent structures should be obtained using the computer, rather than relying on the observer's eyes alone.